1. Field of the Invention
The present invention relates to a radio communication system operated under a digital modulation method, in which a carrier is amplitude modulated according to data to be transmitted.
2. Description of the Prior Art
In a radio communication system for transmitting digital data, a carrier is digitally modulated by data to be transmitted. A variety of methods have been proposed and put into practice for this purpose, for example, the phase shift keying (PSK) modulation method, frequency shift keying (FSK) modulation method, and amplitude shift keying (ASK) modulation method.
In the on-off keying type ASK modulation method, the carrier is transmitted or not transmitted in accordance with data of logic "1" or "0", respectively. This method therefore has an advantage in the extreme ease of its processing operation for transmitting and receiving data. The on-off keying type ASK modulation method has, however, suffered from several problems. First, because the carrier is transmitted or not transmitted during data transmission, the data error rate is higher than that of other modulation methods. Second, it is not easy to distinguish the condition where a carrier is not generated due to, for example, a pause in the system operation from the condition where a carrier is not transmitted due to the transmission of data of logic "0". This makes it difficult to employ the on-off keying type ASK modulation method in a system operated under, for example, a time division multiple access (TDMA) method, in which burst-like data transmission is performed. In the TDMA method, it is not easy to distinguish the condition of burst-like data transmission of logic "0" and the condition of a carrier pause. Third, an automatic gain control (AGC) operation is usually essential in a data transmitting and receiving system for stabilizing the received signal level. In on-off keying type ASK modulation it is not possible to perform an AGC operation when the receiving signal represents logic "0". Further, it is impossible to establish an optimum AGC operation instantaneously for a received signal representing logic "1" i.e., the carrier, appearing immediately after a signal of logic "0", because no meaningful AGC is established for the preceding signal of logic "0".